As used herein, “/” denotes alternative names for the same or similar components or structures. That is, a “/” can be taken as meaning “or” as used herein. Unicast transmissions are between a single sender/transmitter and a single receiver. Broadcast transmissions are between a single sender/transmitter and all receivers within receiving range of the transmitter. Multicast transmissions are between a single sender/transmitter and a subset of the receivers within receiving range of the transmitter where the subset of receivers with receiving range of the transmitter may be the entire subset. That is, multicast may include broadcast and is therefore a broader term than broadcast as used herein. Data is transmitted in packets or frames.
In wireless local area networks, an access point (AP)/station (STA)/mobile device/mobile terminal/node transmits multicast and broadcast frames immediately after it determines that the wireless medium/channel is idle/clear using a physical and virtual carrier sense process with a random backoff time. Therefore, collisions may occur when multiple STAs attempt to transmit simultaneously. For example, an access point and its associated STAs may transmit simultaneously and then all the transmitted frames are lost due to collision. For example, when an AP sends multicast data frames to several of its associated STAs, another STA may send its data or control frames to the AP. The AP with its associated stations is called a basic service set (BSS). Collisions occurring within a BSS are called intra-BSS collisions. In another example, AP1 and AP2 operate on the same channel/frequency and the two BSSs overlap. AP1 transmits data frames to its associated STAs. However, AP2 cannot hear AP1's transmission and thinks that the medium is idle. AP1 is a “hidden node” to AP2. AP2 may transmit frames to its associated STAs while AP1 is transmitting. But there are STAs that are in the interference range of both AP1 and AP2 so that the transmitted frames from AP1 and AP2 are lost at the STAs due to collision. This type of collision is called overlap BSS collision or inter-BSS collision.
In one prior art scheme, a time-division multiple access (TDMA) method was described, where each STA scheduled its transmissions using TDMA time slots (time periods). However, this mechanism required synchronization among the STAs and did not scale well as the number of STAs increased. In another prior art scheme, in order to solve inter-BSS collision, each access point scheduled its multicast and broadcast transmissions in a TDMA fashion using beacons. This method also required synchronization among BSSs.
US 2006/0109859 METHOD AND SYSTEM FOR EFFICIENT AND RELIABLE MAC-LAYER MULTICAST WIRELESS TRANSMISSIONS by Acharya et al. (hereinafter “Acharya”) describes a method for reliable multicasting in a wireless network. A sender node transmits a Request-to-Send (RTS) signal to the receiver nodes. The RTS signal includes a set of identifiers corresponding to the receiver nodes. The Clear-to-Send (CTS) signals and acknowledgment (ACK) signals are sent from the receiver nodes according to an order function of the set of identifiers. In this approach, a data packet is sent from the sender node to the receiver nodes. One or more ACK signals are sent from one or more of the receiver nodes according to an ordering function of the set of identifiers in the RTS signal if a data packet is received correctly. This approach uses a four-way handshake to send a single data packet. It incurs high overhead. In the present invention, a RTS/CTS exchange reserves the channel for transmitting multiple multicast packets. Acharya does not send multiple multicast packets. The method of Acharya multicasts a single packet at a time. That means a great deal of overhead to multicast a single packet. In sharp contrast, the present invention reserves a time period TXOP to send multiple packets of data. That is, the present invention transmits multiple packets within the same TXOP. These packets may or may not be acknowledged. If they are acknowledged, they can be acknowledged together in a block. Furthermore, the reserved TXOP can be cancelled if the sender decides not to transmit after reservation. Therefore, the signaling overhead for transmission is reduced and the network throughput is improved with the present invention.
In another embodiment of Acharya, a bit vector, instead of a set of label identifiers, is included in the RTS. Each bit in the bit-vector indicates a recipient node. However, an AP may have a large number of associated stations, or a sending node may have a large number of neighbor nodes. If the RTS signal includes a set of identifiers corresponding to the receiver nodes or a bit-vector, the RTS signal may become large, leading to more overhead. In sharp contrast, the present invention describes using a bitmap control field and a partial virtual bitmap to reduce the overhead.
It would be advantageous to have a method and apparatus to solve intra-BSS collisions and inter-BSS collisions for multicast transmissions that did not require synchronization, reduce the signaling overhead to transmit data/packets/frames, and improve the network throughput. The present invention solves both intra-BSS collisions and inter-BSS collisions.